Although the toxicity of ultraviolet light has long been recognized, only recently has it become clear that visible light may also be toxic to cell systems. The cutaneous manifestations of certain diseases such as erythropoietic porphyria, urticaria solare (less than 4000 A) and erythropoietic protoporphyria have been shown to be induced by visible light. Recently it has also been shown that visible light can be toxic to bacteria, and that the carotenoid pigments in certain bacteria can exert a protective action against the toxic photosensitization of visible light. Since carotenoid pigments are found in animal cells, the question naturally arises, can these pigments protect against the toxic actions of visible light in animal cells as they do in bacteria? Experiments by the principal investigator with mice have shown that when animals are treated with the photosensitizer hematoporphyrin and develop lethal photosensitization by visible light, carotenoid pigments can be completely protective. In view of the finding that carotenoids can prevent photosensitization in bacteria and animals, we treated three patients with beta-carotene to the point of carotenemia. Because the results we have obtained up to the present time with these patients suggest that the carotenoids might have produced some amelioration of the patients' photosensitivity, we wish to test this hypothesis on a larger series of patients. Thus, we can determine if in actuality the administration of carotenoid pigments is an effective therapy in photosensitive skin diseases. By the end of the third year of this study we have treated 51 patients, only one of whom has failed to show any improvement in light tolerance. Of the remaining 50, over 90 percent have at least tripled the time that they can tolerate light exposure.